A conventional laminated type piezoelectric apparatus is described in Jap. Pat Laid-Open Publication No. 63-62977.
This laminated piezoelectric member is constituted such that the piezoelectric member is expanded or contracted by adding or eliminating a voltage across the piezoelectric member.
A valve means is then driven thereby by varying fluid pressure under this expansion or contraction.
However, piezoelectric devices have a small expansion. This can easily be absorbed by clearances in the device.
Due to this fact, in the prior art device, it is necessary to apply an initial load to the piezoelectric member. When the initial load is not applied or the applied load is insufficient, a generated load and an amount of expansion when the voltage is applied to the piezoelectric element are varied. This results is trouble in driving the valve means.
The conventional type of piezoelectric element disc spring has a large spring constant to apply such an initial load. The initial load is applied to the piezoelectric element by this disc spring.
However, the disc spring described above may have a variation in load, caused by its deterioration as it ages, resulting in a poor durability. In addition, because it is necessary to have a large spring constant, a size of the spring is made large and then an entire size of the liminated type piezoelectric element is increased.
The present invention has been invented in view of the aforesaid points and it is a object of the present invention to eliminate the spring means which has previously been necessary for applying the initial load.
According to the present invention, while voltage is applied to the piezoelectric member, a passage means to the driven means is closed by the valve means. With this arrangement, fluid from the fluid force acting means may apply a fluid force to the former passage means and the piezoelectric member is biased in a direction to press together the laminated plates. This direction is called the laminating or contracting direction. The fluid force is applied through the pressing member.
The fluid force of the fluid acting against the former passage means is held within the passage means by a means for holding the fluid force.
As a result, the pressing force of the piezoelectric member toward the laminating direction through the pressing member is maintained, and an applying of the initial load against the piezoelectric member is accomplished.
The passage means for the driven means is released by the valve means when the voltage is applied to the piezoelectric member and the fluid from the fluid force applying means may cause the fluid force to act on the passage means. As a result, the driven means is driven.
As described above, according to the present invention, it is possible to apply an initial load to the piezoelectric member using the fluid force instead of spring force for accomplishing the driving action of the driven means.
Accordingly, the load will not be unstable as found in case of a spring means due to deterioration due to aging, and a maintaining of a stable load can be attained. In addition, due to an applying of load by the fluid force, a uniform load in respect to the piezoelectric member can be attained.